1. Field of the Invention
The present invention relates to a capacitance circuit, and is mainly concerned with the semiconductor junction capacitance in a semiconductor integrated circuit.
2. Description of the Prior Art
Shown in FIG. 1 is an emitter follower type transistor detector circuit for color television, in which a filter circuit consisting of a capacitor C.sub.1 and a resistance R.sub.1 is connected to the collector of a detector transistor TR in order to prevent high frequencies from leaking to a power source V.sub.cc. In case of constructing such a circuit in the form of a semiconductor integrated circuit (hereinbelow termed "IC"), the capacitor C.sub.1 for the filter has hitherto been formed of a large-capacitance semiconductor junction capacitor which is as shown in FIGS. 2a or 2b and which makes use of the p-n junction between an n.sup.+ -type buried region 4 and a p.sup.+ -type isolated region 5 within a semiconductor substrate member (including layers 1 and 2). With such construction, however, large noises have often arisen.
The reason for such noise has been clarified, and is as stated below. The breakdown voltage of the p-n junction is determined by the impurity concentrations of the n.sup.+ -type buried region and the p.sup.+ -type isolated region. When a voltage exceeding the breakdown voltage is applied to the capacitor C.sub.1 within the IC, breakdown occurs. In the occurrence of the breakdown, large pulsating noises are generated from the capacitor C.sub.1, and the noise characteristic of the IC is degraded. It is generally said that the pulsating noises at the breakdown of the p-n junction are greatly ascribable to the microplasma breakdown which is a non-uniform avalanche phenomenon. The microplasma breakdown starts from a minute breakdown point having a diameter of approximately 1 .mu., and gives rise to current pulses of approximately 10 - 100 .mu.A with irregular widths in a transient breakdown region.
The breakdown voltage of the junction capacitor can be raised by lowering the impurity concentrations of the junction portion. In this case, however, the capacitance becomes smaller for an identical elemental area. In other words, in the case where the capacitor having a predetermined capacitance is to be produced, the area of the capacitor becomes larger. This leads to a larger chip area for the whole integrated circuit, and brings about a lowering of the yield of the IC's in the manufacture.
Furthermore, due to the lowering of the impurity concentrations, such parameters as the collector series resistance r.sub.SC of the transistor within the semiconductor integrated circuit change, so that various characteristics of the integrated circuit vary undesirably.
The present invention utilizes the idea that the noise level of a Zener diode (for which the emitter-base junction of a transistor is exploited, as illustrated in FIG. 3) is inversely proportional to the density of current flowing through the diode and that it has a characteristic as shown in a curve diagram in FIG. 4 (IEEE JOURNAL OF SOLID-STATE CIRCUITS, December 1971, pp. 366-376, "Five-Terminal .+-. 15V Monolithic Regulator"). More particularly, a Zener diode having a Zener voltage lower than the breakdown voltage of the p-n junction capacitor is connected in parallel with the semiconductor junction capacitor, whereby the breakdown of the junction capacitor is prevented to thus avoid the generation of noise from the capacitor. In addition, the junction area is made small, whereby suppression of the noise level of the Zener diode is accomplished.